Many thermoplastic elastomer compositions exhibit a trade-off between processability and physical properties. For example, elastomers with beneficial physical properties such as ethylene-propylene copolymers (EP's), styrenic block copolymers (SBC's), and polyurethanes are difficult to process into useful articles. Among these elastomers, increases in hardness, flexural modulus, and tensile strength are typically accompanied by losses in elastic properties, such as elastic recovery and hysteresis. In particular, SBC's used in elastomeric applications suffer from the disadvantage that they cannot be drawn to desired thicknesses and must be blended with other materials such as ethylene vinyl acetate copolymers, ethylene methacrylate copolymers, or low density polyethylene plastomers to achieve beneficial processability characteristics. However, the use of such processing aids leads to loss of the desirable chemical and mechanical properties of the SBC.
Polyolefin thermoplastic elastomers comprising blends of propylene polymers with non-crystalline ethylene-α-olefin random copolymers or with hydrogenated products of styrene-butadiene-styrene block copolymers are disclosed in Japanese laid-open patent application Nos. Sho 50-14742/1975, Sho 52-65551/1977, Sho 58-20664/1983, and Sho 58-215446/1983. However, elastomeric compositions obtained by blending propylene polymers with SBC's and further with hydrocarbon oils for improving flowability, have a drawback that when molded, the mechanical strength properties, e.g. tensile strength and flexural modulus, of the resulting molded products are lower. Further, elastomeric compositions obtained by blending propylene polymers with non-crystalline ethylene-α-olefin random copolymers have good rubber elasticity, but on the other hand, they have high compounding viscosities.
Similarly, Japanese laid-open patent application No. Sho 61-14248/1986 discloses an elastomeric composition obtained by blending propylene-ethylene block copolymers with SBC's, and if necessary, non-crystalline ethylene-propylene rubbers. However these blends suffer from high compound viscosity leading to difficult fabrication and surface marks.
US 20070240605 (also published as EP 1778781 and WO 06020309, incorporated by reference herein) discloses SBC's used in conjunction with propylene dominated elastomers such that the combinations are found to have desirable elastomeric properties, while at the same time exhibiting beneficial processability characteristics. However, none of the example data disclose SBC formulations having simultaneously a melt flow rate of at least 50 dg/min and a flexural modulus less than 100 MPa. Further improvements in balancing the elastomeric properties and processability characteristics of blends containing SBC are desired.
It has been discovered herein that styrenic block copolymers blended with certain low molecular weight polymer materials incorporating propylene-derived units provide polymeric compositions with an improved balance between processability and elasticity. The improved processability of the compositions arising from the use of these propylene copolymers relates to the ease with which a rubbery material, usually difficult to extrude, can be extruded at high line speeds, which in turn influences the draw-down into films from an extrusion die and is assisted by a desired level of melt elasticity, i.e., a high elastic extension and recovery, even after many cycles. More specifically, in certain embodiments, the polymeric compositions described herein provide additional control of processability, softness and service temperature without significantly detracting from underlying tensile and permanent set characteristics. Most importantly, in blends with styrenic block copolymers the propylene copolymers are able to form soft, malleable and easily processable compositions, whereas similar blends, where process oils are substituted for all or part of the propylene copolymers, form powdery or granular mixtures which are neither malleable nor easily processed.
The propylene copolymers have limited crystallinity resulting from a controlled disruption in isotactic propylene sequences of the polymer, which leads to improved processing of the styrenic block copolymers and retention of mechanical and chemical properties. In addition, these blends may be weld bonded to polyolefin substrates, typically polypropylene compounds or fabrications, more readily than the styrenic block copolymer alone. This improved bonding is believed to be due to better processing, as well as similarity of the composition of the polypropylene substrate and the blend of this invention. Moreover, the polymeric compositions of this invention are soft and elastic yet have improved flow properties at processing temperatures that are better than those displayed by previous materials.